TriggerRunnerSupervisor: Add trigger queue delay metric

[SameerMesiah97]

Description

This change adds a trigger_queue_delay timing metric to measure the time between a trigger workload being queued by the TriggerRunnerSupervisor and being processed by the TriggerRunner.

A queued_at timestamp is recorded when RunTrigger workloads are enqueued, and the resulting delay is emitted as a timing metric during trigger creation. The metric includes the triggerer's team_name tag when available.

Rationale

The triggerer currently provides limited visibility into trigger startup latency. During periods of high trigger creation volume, workloads may spend time waiting before they are processed, but this delay is not currently observable.

This metric helps identify triggerer backlog and provides insight into trigger startup performance under load.

Tests

Added unit tests verifying that the trigger_queue_delay metric is emitted with the expected value and tags when a queued workload is processed.

Backwards Compatibility

This change is additive only and does not modify trigger execution behavior, lifecycle semantics, or public APIs.

[SameerMesiah97]

Requesting review for this.

[potiuk]

@SameerMesiah97 — I've removed the ready for maintainer review label because the next step here is yours: this branch now has merge conflicts with main. It'll go back into the maintainer queue automatically once you rebase onto the latest main and push — no need to re-add the label by hand. There's no rush.

_{Automated triage note drafted by an AI-assisted tool — may get things wrong; a real Apache Airflow maintainer takes the next look once it's green. (why automated)}

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Drafted-by: Claude Code (Opus 4.8); reviewed by @potiuk before posting

[shahar1]

We probably need a newsfragment for this PR.
I think that overall LGTM, but I'll be happy for additional feedback before approval and merging.
@ferruzzi / @o-nikolas - could you please take a second look?